IBC 06 Table 1804.2 Bearing Capacities Allow. or Gross?
IBC 06 Table 1804.2 Bearing Capacities Allow. or Gross?
(OP)
Table 1804.2 in the IBC 2006 Building code gives presumptive soil values based on the class of material. Is the allowable foundation pressure they have listed a net or gorss value? In other words can I neglect the weight of the footing and soil overburden when using these values?
Thanks
Thanks






RE: IBC 06 Table 1804.2 Bearing Capacities Allow. or Gross?
RE: IBC 06 Table 1804.2 Bearing Capacities Allow. or Gross?
RE: IBC 06 Table 1804.2 Bearing Capacities Allow. or Gross?
Having said that I probably would not increase the allowable in an area such as a basement where I was removing the overburden without the recommendation from a geotech.
RE: IBC 06 Table 1804.2 Bearing Capacities Allow. or Gross?
RE: IBC 06 Table 1804.2 Bearing Capacities Allow. or Gross?
He further indicates that the presumptive allowable bearing pressure is also known as the allowable bearing capacity or allowable foundation pressure. This is where I have major problems. We are engineers and should be very clear in our choice of words. In my view and the view by most in the geotechnical community I would surmise is that we have two conditions to consider.
(1) shear conditions - this leads to the ultimate bearing capacity (based on shear) (i.e., about 6xSu for clays). The allowable bearing capacity is the ultimate divided by the factor of safety to be provided (normally 3). This is almost always "gross bearing" as the embedment factor is included in the computation of the bearing capacity (the Df term).
(2) Settlement (or service) conditions - which, for practical purposes is based on settlement considerations (which might be structure mandated, but could be based on other considerations (like expected settlement under seismic loadings)). In such cases, in my view, we should carefully use the term allowable bearing pressure as opposed to capacity (shear). This is almost always based on net bearing pressures.
In essense, "capacity" implies shear or strength and "pressure" implies service or settlement. If we could all adopt this terminology, we would have a far simpler life. Countless hours are probably spent fretting over whether a quoted bearing value is "shear" or "settlement" governed. I would suggest that cleaning up our terminology would give us more time to enjoy other facets of life!
RE: IBC 06 Table 1804.2 Bearing Capacities Allow. or Gross?
This would be highly unlikely as it is allowable to increase the capacity of the soil with depth.
I use the table Value X ( 1 + .2(H -1)) where "H" is the depth to the bottom of the footing. Soils report values would take precidence though and not be increased without the OK of the Geotech.
Mike McCann
McCann Engineering
RE: IBC 06 Table 1804.2 Bearing Capacities Allow. or Gross?
RE: IBC 06 Table 1804.2 Bearing Capacities Allow. or Gross?
RE: IBC 06 Table 1804.2 Bearing Capacities Allow. or Gross?
bottom line: i suggest you get a geotech to check out the site and provide the necessary recommendations.
RE: IBC 06 Table 1804.2 Bearing Capacities Allow. or Gross?
I apologize for this little aside. I have drifted slightly from my original question on gross versus net.
RE: IBC 06 Table 1804.2 Bearing Capacities Allow. or Gross?
and i've seen some rather funny "assumed" values in my relatively short engineering career...
RE: IBC 06 Table 1804.2 Bearing Capacities Allow. or Gross?